1. Field of the Invention
The present invention relates to electric resistance measuring apparatus and methods for circuit boards.
2. Description of the Background Art
In the electrical inspection of circuit boards, it is generally conducted to measure electric resistance between electrodes in the circuit boards.
In the measurement of such electric resistance, there has heretofore been adopted means for finding the value of electric resistance between electrodes 91, 92 to be inspected, which are electrically connected to each other in a circuit board 90 to be inspected, for example, as illustrated in FIG. 28, in which probes PA, PD for current supply and probes PB, PC for voltage measurement are pressed against and contacted with the respective electrodes 91, 92, respectively, a current is supplied between the probes PA, PD for current supply from an electric power unit 93 in this state, and a voltage signal detected by the probes PB, PC for voltage measurement at this time is processed in an electric signal processing unit 94.
However, in the above-described method, it is necessary to bring the probes PA, PD for current supply and the probes PB, PC for voltage measurement into contact with the respective electrodes 91, 92 by considerably great pressing force. In addition, since the probes are made of a metal, and their tips are in the form of a spire, the surfaces of the electrodes 91, 92 are damaged by pressing the respective probes against them, so that such a circuit board becomes unusable. By such circumstances, the measurement of electric resistance cannot be performed on every circuit board as product, so that the measurement cannot but be conducted as the so-called sampling inspection, and the yield of the products cannot be made higher after all.
In order to solve such a problem, there has been proposed an electric resistance measuring apparatus in which elastic connecting members formed of conductive rubber, in which conductive particles are bonded in an elastomer, are respectively arranged on electrodes for current supply and electrodes for voltage measurement (see Japanese Patent Application Laid-Open No. 26446/1997).
According to this electric resistance measuring apparatus, the electrodes for current supply and the electrodes for voltage measurement are pressed against and contacted with electrodes to be inspected in a circuit board to be inspected through the elastic connecting members, thereby achieving electrical connection. Therefore, the measurement of electric resistance can be performed without damaging the electrodes to be inspected.
However, the above-described electric resistance measuring apparatus for circuit boards involves the following problems. Namely, in recent years, the size and pitch or electrode clearance of electrodes have tended to become small in circuit boards in order to high degree of integration is achieved. However, the above-described electric resistance measuring apparatus for circuit boards requires to bring the elastic connecting members related to the electrodes for current supply and the elastic connecting members related to the electrodes for voltage measurement into contact with the respective electrodes in such a circuit board at the same time. Accordingly, when the electric resistance measuring apparatus for circuit boards is used in measurement of electric resistance between electrodes in a circuit board in which electrodes are arranged at high density, it is necessary to form an electrode for current supply as well as an electrode for voltage measurement for each of electrodes to be inspected, which are small in size, and in a state spaced away from each other within a region of an area equal to or smaller than the region occupied by the electrode to be inspected, namely, to form an electrode for current supply and an electrode for voltage measurement, which are smaller in size than the electrode to be inspected, in a state spaced away from each other at an extremely short distance. In addition, it is also necessary to form 2 elastic connecting members independently of each other on the surface of the electrode for current supply and the electrode for voltage measurement, namely, to form elastic connecting members, which are smaller in size than the electrode to be inspected, in a state spaced away from each other at an extremely short distance.
However, it is extremely difficult to do so, and after all, it is extremely difficult to produce an electric resistance measuring apparatus for circuit boards, which is suitable for use in measurement of electric resistance in a circuit board having small-sized electrodes at high density.